1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for measuring an electrical resistivity of a fluid, such as, the borehole mud.
2. Discussion of the Background
During the past years, with the increase in price of fossil fuels, the interest in developing new production fields has dramatically increased. On a drilling rig, mud is pumped from mud pits through a drill string from where it sprays out of nozzles on a drill bit, cleaning and cooling the drill bit in the process. The mud carrying crushed or cut rock is brought back up to the surface through an annular space between the drill string and sides of the borehole being drilled or a casing of the borehole. At the surface, the mud is filtered and returned to the mud pits.
In order to measure electrical characteristics of geological formations drilled through, accurate knowledge of the resistivity of the fluid in the borehole (i.e., the mud) is desired for removing its effects from galvanic, inductive or other measurement techniques.
Conventionally, the mud resistivity is measured using a tool placed in a drill string (i.e., a segmented pipe built as the borehole is drilled). Instantaneous values of mud resistivity are acquired when the mud, passes through an open-end measurement tube of the tool. The measurement of the mud resistivity using the conventional tool is performed using a low frequency alternating current and four electrodes: two outer current electrodes and two central voltage electrodes. According to Ohm's law, a ratio of a voltage measured between the two central voltage electrodes and a current passing therethrough yields a resistance R, which is attributed to a fluid passing through the measurement tube between the central voltage electrodes. The mud resistivity is calculated using the resistance R and known geometrical characteristics of the tool, such as, a distance between the central voltage electrodes and an area of the measurement tube through which fluid passes and which is perpendicular to a current direction.
One problem with the conventional measurement described above is that the resistance R is obtained using an inaccurate value of the current. In fact, the current injected by the outer current electrodes is divided into a part flowing between the current electrodes inside the measurement tube, and a part flowing between the current electrodes through mud in the borehole outside the measurement tube. Thus, some of the injected current is diverted away from the measurement electrodes introducing uncertainty and variability in the measurement.
Some solutions to this problem have been attempted with mixed results, the attempted solutions being affected by additional errors. In one attempted solution, at least one ‘bucking’ electrode at zero (ground) potential has been placed in addition to the four electrodes, outside the four electrodes in a fluid and current flowing direction, to force a potential difference on an electrical circuit through the borehole, outside the measurement tube, to be zero, thereby forcing all the current to flow through the measurement tube. The downside of this method is that it requires a parallel control loop to maintain the voltage correctly on the bucking electrode(s).
In another attempted solution, the measurement current is split into two equal paths that are returned to an electrode placed at a midpoint of the measurement tube. Thus, two separate measurements are performed and any other current still flowing out into the borehole is ignored as it does not return through the measurement apparatus. This technique provides two simultaneous results, without resolving the issue of which one of the results is the more correct.
None of the above attempted solutions has correctly and definitively solved the problem of the current flowing outside the measurement tube. Accordingly, it would be desirable to provide systems and methods that overcome the afore-described problems and drawbacks.